Detectors, such as UV absorbance detectors, are measuring instruments used in liquid chromatography to detect and/or measure constituents of a liquid solution. A flow cell comprises part of a detector and defines the sensor volume of the detector. The flow cell's internal cavity, through which the liquid specimen flows, exposes properties of the liquid to measuring apparatus.
Optical Absorbance
An optical flow cell allows transmission of ultraviolet or visible light through liquid flowing therein. The liquid's absorbance can be measured for the purpose of detecting the presence, and determining the concentration, of certain chemical compounds, such as species of protein, DNA or small drug molecules. Absorbance is calculated using Beer-Lambert's law:A=εbc  (1)
Where A is absorbance (unitless);
ε is molar absorbtivity of the analyte (L mol−1 cm−1)
b is optical path length (cm);
c is concentration (mol L−1).
Also,
                    A        =                              log            10                    ⁡                      (                                          P                0                            P                        )                                              (        2        )            
Where P0 is the incident radiant flux;
P is the transmitted radiant flux.
The Optical Flow Cell
The optical flow cell is typically defined by a bore in the interior of a body, comprising a fluid inlet and fluid outlet and establishing the flow path for liquid wherein absorbance is measured. The optical flow cell body typically includes a second bore, defining an optical path which allows light to pass through at least a portion of the liquid flow path. A portion of the liquid flow path is shared with the optical path and usually runs parallel to the optical path. Windows, with the aid of a sealing gasket, seal against the ends of the second bore, allowing light to pass while retaining the liquid inside the optical flow cell body. The windows are often made from fused silica or another material optically transparent at the measured wavelengths, compatible with the liquid and sufficiently strong to withstand the internal flow cell cavity forces. Typically, the detector body includes one or more grooves or slots on each end of the optical path where the optical path end face joins a window. The groove directs fluid into one open end of the optical path and out the opposing end. This combination of geometric features promotes liquid flow through the entire optical path and minimizes mixing of the currently passing liquid with liquid that had flowed through the cavity earlier. Typically, a window sealing gasket made from a compliant material compatible with the liquid, like PTFE (DuPont Teflon®) or an elastomeric material like perfluoroelastomer (DuPont Kalrez®), silicone or rubber (Buna-N) is disposed between the window and the cell body as a seal, retaining the liquid in the flow cell.
Multi Wavelength Detection
In liquid chromatography systems, it is often necessary or desirable to measure the liquid solution's absorption at more than one optical wavelength. Light absorption detectors often employ a broad spectrum light source coupled to a monochromator to select each wavelength of interest. Other light absorption detectors use a broad spectrum lamp and a holographic grating to split the transmitted light into constituent wavelengths, then measure the transmitted wavelengths with a linear array of light sensitive elements such as photodiodes or charge-coupled devices (CCDs).
Conductivity
Electrolytic conductivity of a liquid solution is often related to the ionic concentration of a solution. Measured in conjunction with light absorbance, conductivity provides additional information about the liquid, such as salt concentration and protein concentration. Conductivity is usually determined by measuring the resistance across two electrodes. Dilute solutions, typical of liquid chromatography, follow Kohlrauch's law:Λm∞=vcationλcation∞+Vanionλcation∞  (3)                Where Λ∞m is the molar conductivity of an electrolyte;        Vcation and Vanion are the numbers of cations and anions per formula unit respectively;        Λ∞cation and λ∞cation are the molar conductivities of the cation and anion at infinite dilution respectively.Conductivity Flow Cell        
Typical conductivity flow cells include two metal disks including holes in their centers through which the liquid solution flows. The metal disks are separated by an insulator, preventing electrical conduction between the optical flow cell bodies other than through the liquid solution. Thereby, the disks act as electrodes for measuring the conductivity of the liquid.
In some cases, there is a substantial amount of current flow, not due to the liquid's conductance, present in the measured signal. These erroneous currents are due to inherent properties of the electrodes, like capacitance and fringe effects.
Two conductive surfaces separated by an insulator form a capacitor. The two conductivity electrodes meet this definition and, therefore, form a capacitor. The capacitance of the conductivity electrodes affects the conductivity readings.
Some electrical current flows through the longest path through the fluid. Other current flows through the shortest path. These variations in path length cause fringe effects, a broadening of the conductivity reading.
Pairing Optical and Conductivity Flow Cells
Conventional liquid chromatography systems contain separate flow cells for measuring optical absorbance and for measuring electrolytic conductivity. Furthermore, said systems may contain several flow cells in series, each measuring a single property of a liquid specimen solution. It would be desirable to reduce the number of flow cells in a liquid chromatography system for several reasons. Flow cells take up space in compact liquid chromatography systems. Flow cells add to the length and complexity of the liquid flow path. Multiple flow cells connected in series adds to the number of liquid connection points, increasing the likelihood for liquid leaks or obstructions to smooth liquid flow. Moreover, flow cells naturally increase liquid solution turbulence, causing errors in absorbance and conductivity measurement. Most importantly, the internal volume of these flow cells and their interconnections form volumes for previously separated compounds to remix, reversing the effects of the chromatographic separation process.
The Combined Sensor
Accordingly, it would be desirable to provide a flow cell that can perform measure the physical properties of the liquid solution and substantially overcome the disadvantages and problems encountered by employing multiple separate flow cells. The present invention provides such flow cells, methods for their use, and liquid chromatography systems including such flow cells.